The present invention relates to a device for use in the manufacture of fiber pulp from lignocellulosic material, such as wood chips, comprising a refining apparatus with a refining gap for the comminution of the material between mutually rotatable refining elements, and a supply line for feeding the material into the refining gap. The invention is conceived in the first place for use in conjunction with refining apparatus of the type in which the refining elements are mutually rotatable discs subjected to thrust pressure and having a central supply duct to carry the material to the refining gap between the discs. However, the invention is not limited to this application, but can also be applied, for example, to refining apparatuses of the cylinder type, where the refining gap is formed between an outer cylindrical drum and a rotor that rotates inside the said outer drum. Of the energy supplied to the rotating element of the refining apparatus for the purpose of comminuting the material into fibers or fibrils, a large part is converted into heat, whereby steam is generated in the refining gap by the vaporization of the water accompanying the material. The volume of steam generated with the high energy inputs required by modern designs is very large, and to prevent the dry content of the material hereby reaching excessive values, it is a known practice to add water into the refining gap during the refining process in order to maintain in the desired water content of the material, thereby protecting the latter from damage by overheating.
It is a known practice to permit part of the generated steam to blow back into the fiber material supply line leading to the refining gap, and also to utilize this steam for pre-heating the material in a pre-heater before it is introduced between the refining discs. Another portion of the steam generated in the refining process passes along with the material through the gap between the refining discs, and is fed out therefrom together with the fiber pulp product. This implies that a zone of maximum steam pressure arises in the refining gap, being located somewhere near the middle of the refining space, with steam flowing in both directions from the said zone.